Complementary metal oxide semiconductor (CMOS) transistor devices currently are in widespread use in a variety of applications for low power electronic components. Low power CMOS oscillators are particularly popular for watches and the time/date clock circuits used in personal computers. For both of these applications, it is desirable to lower the power consumption of the electronic circuitry as much as possible to permit operation of such watches and clock circuits for long periods of time from batteries of small size and relatively low energy capacity. Popular circuits for attaining such desired results typically utilize an oscillator created using a CMOS inverter with a resonant circuit coupled between the output and input terminals. The resonant circuit, for electronic watches and computer clocks, also includes a quartz crystal operated at a resonant frequency of 32,768 Hz (approximately 32 KHz) to provide the time base. The output of the oscillator then is supplied to a processing circuit, including frequency dividers, to control a digital display or to drive a stepping motor (in the case of an analog watch) to produce the desired system output.
When a quartz crystal is operated at the relatively low frequency of 32 KHz, the physical construction of the crystal is somewhat fragile. Typically, the crystal is in the form of an elongated rod mounted at one end in a "tuning fork" configuration. The relatively long thin crystal rod requires precautions to be taken to prevent overdriving the crystal with too much power. If such a fragile crystal is overdriven, it is subject to permanent damage. In the case of electronic watches, the battery which is used to supply power to the circuitry typically has a voltage on the order of 1.2 volts. This voltage is sufficiently low that overdriving of the crystal is not possible. For computer internal clock applications, however, the standard direct current power supply typically is of the order of 4.5 to 5.5 volts; and in many currently available computer, a battery back-up supply of 1.8 to 3.5 volts also is available. The standard power supply voltage is sufficiently high that it is possible to overdrive the quartz crystal employed for the time base of the clock circuitry.
Efforts have been made to reduce the power consumption of CMOS oscillators to a point where such an oscillator used in a quartz watch, for example, is capable of operation for a number of years from a small, low-energy battery producing a 1.2 volt supply voltage. Circuits for minimizing the power consumption of CMOS crystal oscillators have been developed to obtain maximum voltage swings, while preventing the transistors of the oscillator circuit from achieving a full turn-on status. In some applications, this is accomplished by providing biasing voltages developed through the use of a current mirror, so that the biasing of the oscillator inverter transistors is independent of processing variables and ambient temperature. In other systems, current sources are connected in the gate circuitry of both of the inverter transistors also to provide an independent biasing of the transistors.
The systems of the type mentioned above usually operate only from a relatively low battery voltage (1.2 volts), so that no provisions are made to prevent to overdriving the crystal, since such overdriving is an impossibility with such low power circuits. If, for some reason, the power supply voltage were increased, there is nothing in such systems to prevent overdriving the crystal.
For applications operating 32 KHz crystal oscillators from higher voltage supplies, it typically has been the approach to utilize a voltage regulator to ensure that the voltage supplied to the CMOS crystal oscillator circuit is sufficiently low to prevent crystal overdriving. For computer applications or the like, typically a 5 volt power supply is employed. A voltage divider including an appropriate number of P-N series-connected diodes, Zener diodes or the like is employed to provide the regulated voltage to the circuit. Such voltage regulator circuits, however, require relatively large resistors, in order to limit power consumption and essentially only work with relatively high voltages (of the order of 5 volts or more). For the back-up battery power supply used for the clock/date circuits of microprocessors or personal computers, the back-up battery voltage usually is of 3 volts or less. If a circuit using a conventional voltage regulator is switched over to the power supply of such a low-voltage back-up battery, the available voltage for the system is too low for a conventional voltage divider voltage regulator. Thus, it is necessary to add additional amplification stages, and therefore increased complexity and cost, to achieve the desired results.
Another problem which exists with low power CMOS crystal oscillators operated at low frequencies (for example, 32 KHz, is that the start-up time for such oscillators is very slow, because of the limited gain/bandwidth of the amplifier inverter.) This condition is particularly aggravated at low voltages, and it is possible that oscillation start-up may not occur at all when a low power system initially is turned-on or powered up.
It is desirable to provide a low power CMOS crystal oscillator in which the gain/bandwidth of the inverter amplifier is increased during start-up and then is cut back to an optimized operating level to permit operation of the oscillator over a range of power supply voltages without overdriving the crystal.
In addition, it should be noted that prior art circuits which employ a voltage divider from a full 5 volt power supply in a clock/date oscillator circuit for a personal computer typically use between five and ten microamps of current for operation. It also is desirable to provide a circuit which achieves the same clock operation at significantly lower levels of power consumption.